Conventionally with respect to, for example, a vehicle engine, so-called exhaust gas recirculation (EGR) has been employed in which part of exhaust gas is extracted from an exhaust side and is returned to an intake side as EGR gas which suppresses combustion of fuel in the engine and lowers a combustion temperature, thereby reducing NOx generated.
Generally, in order to conduct such kind of exhaust gas recirculation, a position in an exhaust passage extending from an exhaust manifold to an exhaust pipe is connected to a position in an intake passage extending from an intake pipe to an intake manifold by an EGR pipe for recirculation of the EGR gas therethrough.
The EGR pipe may be equipped midway thereof with a water-cooled EGR cooler since cooling, midway of the EGR pipe, of the EGR gas to be recirculated to the engine will drop a temperature of and reduce a volume of the EGR gas to lower a combustion temperature without substantial decrease in output of the engine, thereby effectively suppressing generation of NO (nitrogen oxides).
On the other hand, on-board fault diagnosis equipment called OBD system (On Board Diagnostic System) may be boarded on a vehicle nowadays such that the vehicle itself may detect and monitor any fault (sudden malfunction) of exhaust emission control equipment, make alarm display of any occurring fault to a driver and store particulars of the fault.
A prior art reference for such kind of technique for detecting any characteristic fault of a sensor is, for example, the following Patent Literature 1 by the same applicant as that of the present invention.